It is well known in the Nylon industry that adiponitrile (ADN) can be hydrogenated catalytically to produce hexamethylenediamine (HMD) by complete hydrogenation, or mixtures of 6-aminocapronitrile (ACN) and HMD by partial hydrogenation. The hydrogenation reaction product also contains unreacted ADN and unwanted byproducts such as tetrahydroazepine (THA). After hydrogenation, the reaction product must be refined, generally by methods involving fractional distillation, and HMD and ACN must be separated from each other.
It is also known that if the refining conditions involve too high a temperature, the unreacted ADN can isomerize into CPI (2-cyanocyclopentylideneimine). The CPI generally distills with the ADN, and if the CPI/ADN mixture is recycled back to the hydrogenation reactor, the CPI can form AMC (2-aminomethylcyclopentylamine), which, if unseparated from the HMD, can cause inferior Nylon 6,6 to be made.
A solution to the problem is disclosed in U.S. Pat. Nos. 6,346,641 and 6,462,220 that teach distillation processes in which the column temperatures are kept below 185 deg C. However, none of these patents teach methods which allow distillation to be performed in a manner in which HMD can be recovered substantially free of THA.
U.S. Pat. No. 6,300,497 B1 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.3 and 3.0 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.1 and 1.3 bar. U.S. patent application Ser. No. 2003/0023083 A1 teaches a method for reducing the THA content of a THA/HMD mixture by distillation using column head pressures between 0.001 and 0.3 bar, as well as reducing the THA content of a THA/ACN mixture by distillation using column head pressures between 0.001 and 0.2 bar. However, neither of these teach a method in which a three component ACN/HMD/THA mixture is distilled so that the ACN and the HMD can be separated from one another in such a way that a substantial portion of the THA remains with the ACN, particularly when the three component ACN/HMD/THA mixture is one that is derived from the product that is produced by the partial hydrogenation of ADN, such a product containing unreacted ADN, that is capable of being isomerized into undesirable CPI if distillation temperatures in the refining train exceed about 195 deg C.